1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with an improved top nozzle for a fuel assembly incorporating a non-peripheral diagonal arrangement of leaf spring assemblies.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle.
In the conventional fuel assembly, coolant flowing upward past the fuel rods and guide thimbles induces significant upward forces. These forces are countered by a combination of the weight of the fuel assembly and a resiliently yieldable hold-down device of the top nozzle which pushes against the upper core plate of the reactor. The hold-down device thereby provides a downward force which counteracts and prevents the force of the upward coolant flow from lifting the fuel assembly into damaging contact with the upper core plate, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like.
One prior art hold-down device employs a single large diameter helical coil spring centrally in the top nozzle to hold down the fuel assembly. The use of a single coil spring in this device presents two principal problems. First, the coil spring has a tendency to fracture and fail due to vibration induced by coolant flow through the fuel assembly. Second, because of its susceptibility to failure, the coil spring cannot withstand the large loads imposed on it when the flow of coolant increases due to activation of the final, or fourth, reactor coolant pump. As a result, the nuclear power plant operator must refrain from activating the last pump until the coolant reaches its start-up temperature of 450.degree.-500.degree. F., when the density of the coolant is less than at a lower temperature. The pumps heat the coolant by putting mechanical energy into it; thus, it takes longer to heat up the coolant if all the pumps cannot be used from the beginning of the critical path (the time it takes to reach operating temperature). The increase in the time spent to heat up the coolant results in considerable increase in operating costs to the plant operator.
It is known heretofore to use a peripheral arrangement of leaf spring assemblies instead of coil springs in a top nozzle hold-down device to provide the necessary hold-down force to prevent lifting of the fuel assembly, while at the same time accommodating thermal expansion of the assembly. U.S. Pat. Nos. 4,671,924 to Gjertsen et al and 4,684,502 to Wilson et al and the above cross-referenced U.S. patent application, assigned to the assignee of the present invention, U.S. Pat. No. 4,792,429 to Hatfield, and Japanese patent document Nos. 62-102186 and 62-91891 all disclose peripheral arrangements of leaf spring assemblies. Particularly, in the Gjertsen et al and Wilson et al patents, the leaf spring assemblies are aligned along and engaged between opposing peripheries of the upper hold-down plate and lower adapter plate of the top nozzle. These peripheral arrangements of leaf spring assemblies are undoubtedly satisfactory for the particular design of the top nozzle structure with which they are used. However, use of a peripheral arrangement of leaf spring assemblies to replace the single central coil spring of the one prior art hold-down device described above would not solve the above-cited problems without a substantial redesign of the overall top nozzle structure.
Consequently, a need exists for an alternative arrangement of leaf spring assemblies to replace the central coil spring and overcome the problems associated with it without requiring a significant modification of the overall top nozzle structure.